Look in /etc/yum.repos.d/CentOS-Base.repo for the mirrorlist= line in the [base] section and take the URL from that and plug it into curl "http://whatever.the.url/...", first substituting the variables so that $releasever = 6 and $arch is x86_64. Check the output from that - it should be a list of mirrors to use. I suspect you're hitting something like a web proxy that's returning a request for authorization.